The metalloprotease secreted by Bacillus anthracis is part of a two-part toxin that specifically invades macrophages. The enzyme is completely non-toxic in the absence of the second component, protective antigen. Its mode of action is unclear, although recent data suggest that it may activate MAP kinase by cleaving the N-terminal portion. The protein, of MW 85 kDa, is of unknown three-dimensional structure. Its structure would be of great interest in the development of protein delivery systems and vaccine design, and we have already determined the crystal structure of the receptor binding and protein translocation moiety, the protective antigen [Petosa et al. (1997)]. We have crystals of the metalloprotease that diffract to a resolution of about 2.5 A on a conventional synchrotron source, and which contain a dimer (2 x 85 kDa) in the asymmetric unit. We have carried out an exhaustive heavy atom derivative search, and have finally identified a Pt derivative with at least two sites. The derivative is not closely isomorphous with the native, however, so that we have not been able to generate high-resolution phases. By collecting complete MAD data sets we would envisage producing a high quality electron density map into which we could build a complete atomic model of this protein.